A common requirement in radio frequency (RF) circuits is the control of RF signal levels. For example, often in RF systems, such as CATV cable television systems, RF signal levels vary significantly resulting in unpredictable and/or undesired operation of particular components thereof, such as receivers, tuners, repeaters, and the like. Accordingly, such systems often utilize controllable signal attenuators, such as at the input stage of one of the aforementioned components, in order to provide a relatively constant RF signal level as provided to such components.
Often the above mentioned controllable attenuators are provided with a voltage controlled RF attenuator such as a linear attenuator. A linear attenuator typically has three ports or interfaces; those being an RF input port, an RF output port, and a control input. Ideally, a linear attenuator provides attenuation (generally expressed in decibels) between the RF input and RF output ports that is a linear function of a control signal. Other desirable attributes of a linear attenuator may include maintaining a good impedance match at the RF ports with respect the circuit coupled thereto over the control and frequency range, providing a flat attenuation response over a wide band of frequencies, introducing little or no excess noise into the circuit, and generating little or no distortion in the signals attenuated thereby. In RF systems that operate with signals of more than one octave of RF spectrum (broadband) the attenuator must also ensure that, in addition to acting as an attenuator, the RF impedance (return loss) of both the input and output of the attenuator is held as close as possible to the desired system impedance. Failure to maintain a proper impedance match can greatly affect the system frequency response (power transfer) and noise figure.
However, prior art linear attenuators generally provide a tradeoff with respect to these desirable attributes and, therefore, often provide less than ideal operation in demanding system applications. For example, there is generally a trade off between providing a flat attenuation response across a broadband signal and maintaining a good impedance match throughout the control and frequency range. Similarly, previous attenuation circuit implementations have experienced a trade off between providing attenuation that is a linear function of the control input and providing a low insertion loss. Specifically, PIN diode attenuator circuits are available that will provide a decibel per volt linearization, but typically will have a minimum of approximately 3 to 4 dB insertion loss.
One common implementation of a linear attenuator consists of a two section embodiment including a PIN diode attenuator section and a linearizer section coupled to the PIN diode attenuator section. In such a configuration, a PIN diode network, such as a π network or a bridge T network, and passive bias components form the PIN diode attenuator section and provide attenuation of signals passed therethrough in response to a control voltage. Specifically, the PIN diodes exhibit a variable RF resistance that is inversely proportion to the DC current through the diode and, therefore, the arrangement of PIN diodes and the corresponding bias components provides a circuit in which variable attenuation is achieved in response to a control voltage applied to bias components.
Such a PIN diode attenuator transfer function of RF attenuation versus DC current is non-linear due to the non-linear RF resistance of the PIN diodes versus bias current. Accordingly, a linearizer section is provided to allow a linear control voltage applied to an input of the linearizer section to result in a corresponding linear attenuation response of an RF signal applied to the PIN diode attenuator section.
Next-generation digital cable set-top boxes, such as those conforming to the OPENCABLE tuner specifications from Cable Television Laboratories, Inc., must provide attenuation in a large dynamic range (gain control range), such as on the order of 30 dB of dynamic range and beyond, while maintaining the RF input impedance of the device, such as 75 ohms. However, PIN diode attenuator configurations, such as those described above, have heretofore been unable to adequately address such requirements. For example, previously known bridge T attenuator structures are precluded for use in the above conditions as 30 dB of dynamic range are not available with commercially available PIN diodes in the prior art bridge T network configurations. Similarly, previously known π attenuator structures, although perhaps able to achieve a relatively large dynamic range, generally are not able to maintain the return loss or impedance match over this dynamic range. For example, typical prior art π attenuator structures result in poor return loss with designs with greater than 15 dB of attenuation range.
Accordingly, a need exists in the art for a controllable attenuator circuit which provides an excellent return loss over a relatively large attenuation range.